1. Field of the Invention
The present invention relates to a combining diversity apparatus with a squelch function in a digital multi-valued modulation signal receiver and transmitter circuit.
2. Description of the Related Art
In a digital communication system, diversity technology has been used to reduce a rate of transmission errors caused by fading. For example, two antenna units for the same modulation signal are located separately at different sites. This is called space diversity. Then, the two signals received by the two antenna units are combined by an equal-gain combining diversity method or the like. The equal-gain combining diversity method or the like is carried out by hardware or software.
Also, in a digital communication system, a squelch function is introduced into the modulation signal receiver and transmitter circuit, thereby preventing it from erroneously operating to generate a spurious signal therefrom. For example, when an undesired signal or an interference signal is received, such a signal is detected in accordance with a clock signal component included in a received signal, to carry out a squelch operation (see JP-A-HEI4-345229). Also, such a signal is detected in an identification signal component included in the received signal, to carry out a squelch operation (see JP-A-HEI3-116376). Further, when a reception level of a normal signal is extremely reduced, the reduction is detected to carry out a squelch operation (see JP-A-HEI3-58630).
A prior art combining diversity apparatus with a squelch function is constructed by at least first and second receiving antenna units having a space diversity relationship, first and second frequency conversion units for converting the modulation RF signal into first and second IF signals, a combining unit for phasing-combining the first IF signal with the second IF signal, and a squelch unit connected to the combining unit. That is, when an output signal level of the combining unit is deviated from a threshold value, for example, when this output signal level becomes higher than the threshold value, a squelch operation is carried out by the squelch unit, to avoid generation of a spurious signal. This will be explained later in detail.
In the above-described prior art combining diversity apparatus, however, even when undesired waves (interference waves) are introduced into a part of the receiving antenna units, the squelch unit may carry out a squelch operation, so that a combining operation is not carried out normally.